This invention relates to modified lignosulfonate materials and a process for their preparation. These modified lignosulfonates result from the sulfonation of purified lignosulfonates by reaction with sulfite-bisulfite salts. The removal of sugars and other low molecular weight impurities provides a greater concentration of these modified lignosulfonates. These materials are useful in preparing oil well drilling fluid dispersant products with significantly improved performance and thermal properties.
A well fluid for use in rotary drilling must have sufficient viscosity that it easily carries rock chips and material loosened by the drill bit out to the surface of the ground by flow of the fluid, and it should be thixotropic so that when drilling is stopped at any time, the fluid will gel and prevent chips from settling around the drill bit. The apparent viscosity or resistance to flow of drilling muds is the result of two properties, plastic viscosity and yield point. Each of these two properties represents a different source of resistance to flow. Plastic viscosity is a property related to the concentration of solids in the fluid, whereas yield point is a property related to the interparticle forces. Gel strength, on the other hand, is a property that denotes the thixotropy of mud at rest. The yield point, gel strength, and, in turn, the apparent viscosity of the mud, commonly are controlled by chemical treatment with materials such as complex phosphates, alkalies, mined lignites, plant tannins, and modified lignosulfonates. It has been found that chromium modified lignosulfonate as well as a mixed metal lignosulfonate of chromium and iron are highly effective in controlling the viscosity of drilling fluids. Because chromium is a heavy metal, there is concern about its dispersion in the natural environment. Its status is continually being reviewed by various governmental agencies. Various governmental authorities around the world contemplate stringent controls upon the use of compounds containing chromium in oil and gas well drilling lest the fluids containing these agents inadvertently contaminate the environment.
Lignosulfonate derived oil well drilling fluid dispersants have been extensively studied and used. Such dispersants have been produced from the oxidation of thermally treated lignosulfonates in the presence of metal salts of chromium, iron, aluminum and copper. See for example U.S. Pat. Nos. 2,935,473 and 3,087,923, both to King et al. Lignosulfonates without modification have been used to produce titanium and zirconium based dispersants as shown in U.S. Pat. No. 4,220,585 to Javora et al. Subsequent dispersants were derived from fractionated and fermented spent sulfite liquors in conjunction with certain of the aforementioned metals, as shown in U.S. Pat. Nos. 3,244,623 and 3,278,425, both to King et al. The desugaring of lignosulfonates for use as dispersants by reaction with tetravalent sulfur compounds (e.g., SO.sub.2) is disclosed in U.S. Pat. Nos. 3,505,243 and 3,668,123, both to Steinberg et al. and assigned to the present assignee. Ultrafiltration of spent sulfite liquors using equipment developed by De Danske Sukkerfabrikker (DDS) is covered by a number of foreign patents. U.S. Pat. No. 3,244,623 also discloses dialysis or fractionation for removing carbohydrates (i.e. sugars). However, simple dialysis is not sufficiently rapid to be used in commercial filtration of spent sulfite liquors.